1. Field of the Invention
The invention relates to a method of continuously producing a twin-wall pipe comprising a smooth internal pipe and an external pipe that is united with the internal pipe by welding and provided with elevations; a pipe socket; and a central longitudinal axis; the method comprising the following steps: extruding an external tube concentrically of the central longitudinal axis; corrugating the external tube with elevations and troughs by partial vacuum applied from outside; extruding an internal tube into the external tube concentrically of the central longitudinal axis; welding together the internal tube and the troughs of the external tube; expanding the external tube at given distances by the partial vacuum being applied from outside, to form an expanded area for a pipe socket to be produced; actuating the internal tube inwardly by gas of a pressure above atmospheric pressure and expanding and pressing the internal tube full face against the expanded area of the external tube for the pipe socket to be finished; and forming a transition portion between the pipe socket and an adjacent trough, the transition portion being comprised of the internal tube and external tube and directed outwards in relation to the central longitudinal axis. The invention further relates to a twin-wall pipe comprising a central longitudinal axis; a smooth-wall internal pipe which extends concentrically of the central longitudinal axis; a corrugated external pipe, having elevations and troughs between the elevations, the troughs and the internal pipe being welded together; a pipe socket which is molded integrally with the internal pipe and the external pipe; a transition portion between a trough and the pipe socket, the transition portion being formed between the external pipe and the internal pipe and having an inside. The invention further relates to an apparatus for implementing the method and for the producing the twin-wall pipe.
2. Background Art
U.S. Pat. No. 5,320,797 describes a method, twin-wall pipe and apparatus of the generic type. The greater the nominal widths of corrugated pipes, the more grow the elevations and thus the increase in size of the pipe socket in relation to the inside diameter of the twin-wall pipe. This is due to the fact that the standard twin-wall pipe is very often used as a spigot, meaning that a twin-wall pipe is inserted by its elevations into the socket. The transition portions between the twin-wall pipe that leads during in-line production and the pipe socket on the one hand, and the pipe socket and the lagging twin-wall pipe on the other, possess considerable radial extension. In particular the transition portion between a twin-wall pipe and socket, which remains after separation of the extruded continuous run of pipe, must possess pronounced radial extension i.e., must be directed steeply outwards in relation to the central longitudinal axis, so that, upon insertion of the spigot into the socket as far as to the transition portion, there will be no dead space, nor considerable dead space, where dirt might deposit. The greater the nominal widths and/or the higher the production rate, the greater the risk that the internal tube does not adhere by its full face to the external tube in the vicinity of the transition portion and at the beginning and end of the socket.